Another interpretation of microbivory in amphibians, w.r.t. specialisation for metamorphosis
(writing in progress)
@alexanderr Would you like to comment, at the draft stage?
Here is an original idea on the biology of Amphibia, which may amount to a theory rather than just a limited observation. The topic overlaps that of my Post on microbivory in frog larvae = tadpoles.
The two main Orders in the Class Amphibia are Urodela (salamanders) and Anura (frogs). Frogs are widespread on mainlands, whereas salamanders are essentially restricted to the Northern Hemisphere.
The larvae of frogs are extremely different from the adults.
However, in salamanders the larvae are more similar to adults. Furthermore, in salamanders there are various blurrings of the metamorphic sequence, such as
- paedomorphosis (in which the animal reaches sexual maturity while still at the larval stage), and
- the occurrence of ‘efts’ (in which the larva metamorphoses into a terrestrial stage but then returns to water, with corresponding morphological changes, in full maturity).
Given the many different lineages of salamanders which show paedomorphosis, the complete absence of paedomorphosis among frogs is a remarkably categorical difference between the two Orders.
The larvae of frogs tend to eat microbes. However, the larvae of salamanders just eat small animals (including some filter-foraged zooplankton consisting of extremely small animals rather than microbes). Some salamander larvae (e.g. Notophthalmus, Ambystoma) do eat some algae, but algae do not ever seem to dominate the diet in any salamander at any stage of its growth.
Please note in particular that tadpoles tend to be extremely different from adult frogs in their dentition and gut form. Tadpoles tend to have chitinous teeth (found in no other vertebrates) and long, coiled intestines. Larval salamanders lack these features.
The picture that emerges, while well-verified by the literature, has not previously been stated clearly as I am stating it here:
There is a correlation, in amphibians, between microbivory and the degree of metamorphosis.
The above statement has been ‘hiding in plain sight’ in amphibian biology.
To make the pattern clear in its simplest form:
Let us imagine a Rana and a Notophthalmus living in a wetland in North America, which is a common situation.
The Rana has tadpoles that eat microbes. The tadpoles metamorphose radically into terrestrial adults, representing one of the greatest morphological shifts in ontogeny known in any living vertebrate. Indeed, in some frogs, albeit not Rana, the tadpoles actually grow far larger than the adults, metamorphosis involving an actual shrinking of the body – almost as if two GUILDS are ‘born’ under a single species-name, as opposed to babies that just grow and develop into adults.
(Eels, such as Anguilla, have an even more radical shift. It is interesting that the larval stage of eels is also essentially microbivorous, in a sense.)
Note my implication, which you will not see in the literature: that the clear and obligate complete metamorphosis of the Rana is BECAUSE the larva is microbivorous.
In the same wetland, Notophthalmus also has a larval stage and metamorphosis. The larva does eat algae to a limited extent. However, the whole picture is surprisingly different from that in frogs.
Firstly, the larval and adult stages are not particularly different, apart from growth in body size.
Secondly, the terrestrial stage is only temporary (called an eft), before the still-growing adult returns to water.
Furthermore, under certain conditions the eft stage can be skipped, and/or the animal can reach sexual maturity without metamorphosing (i.e. while still retaining its larval gills).
In salamanders, metamorphosis is rather nebulous and facultative and never represents the radical (categorical) morphological switches exemplified by the frog Rana. The variations I have described can all occur within one species of Notophthalmus.
And I am suggesting, apparently for the first time, that this is BECAUSE larval salamanders do not specialise as tadpoles tend to do on microbes in their diet.
To put my new idea as succinctly as possible: eating microbes as a staple DRIVES metamorphosis in some way beyond the purely mechanistic. Or, even more to the point: the existence of tadpoles – tantamount to a kind of morphological duplicity within the species – is only possible in the first place because of microbivory.
Tadpoles are neither ‘herbivorous’ nor ‘grazers’ as so commonly claimed. However, tadpoles would not even exist were it not for the fact that they are microbivores, sharing a guild with microbivorous fishes.
Now things can get interesting, biogeographically.
One of the biggest puzzles in amphibian biology is why southern locations, such as Patagonia, Tasmania and New Zealand, lack salamanders completely.
Salamanders are extremely widespread in the Northern Hemisphere (even crossing the Arctic Circle into tundra) and evolved so long ago that continental drift should not have prevented their dispersal through Gondwana. Caecilians, another Order of amphibians which may be as ancient as Urodela, today reach as far south as Buenos Aires Province and the Tanzanian coast south of the equator.
My new idea is that, in southern lands, certain frogs are the ecological equivalents of salamanders in developing out of the water, which means that they lack a tadpole stage, and thus lack a microbivorous stage.
The small South African frog, Arthroleptis, which is abundant in the fire-free coastal forests of Kwazulu-Natal, is an example of a ‘salamander-equivalent’. This idea may not sound particularly radical, because some salamanders do develop out of the water, much as many frogs do.
What is new is my suggestion that frogs should be divided ecologically on the following basis:
Any frog lineage that develops entirely out of water, i.e. without hatching into a tadpole, is effectively excluded from microbivory, and thus functionally analogous with salamanders rather than with ‘typical’ frogs. Many, many taxa of frogs worldwide are like this.
An essential idea here is as follows:
A frog developing through the ‘tadpole’ stage, within the egg, is NOT actually a tadpole. Instead, it is just an embryo.
It is easy to confuse tadpole and embryo in frogs that lack free-swimming larvae. However, my point is that we should not call it a tadpole, if it is not a microbivore.
The frogs that develop to the legged, adult form within the egg, such as South African Arthroleptis, lack not only an aquatic stage but also a tadpole. (A tadpole is more than a larva and more than a developmental stage: a tadpole is by definition a microbivore.)
What the larva in the egg, which has loosely been thought of as a tadpole in the literature, really eats is yolk. Yolk may be similar to microbes in nutritional value and ‘aggregate unicellularity’, but a tadpole can by definition not eat yolk.
With this conceptual framework in mind, what I would like to see is a new world map of amphibian distributions, referring just to those frogs that develop out of the water. Examples include the blaasops (Breviceps), and the relatively obscure genera Arthroleptella and Anydrophryne, in South Africa.
Breviceps belongs to the Microhylidae and the other two genera belong to the Pyxicephalidae; Athroleptis belongs to the Arthroleptidae. Adults of Arthroleptis have a staple diet of terrestrial amphipods.
Arthroleptella develops on damp moss near fast-flowing streams. Some species at least of Breviceps lay the eggs, out of water, in a foam mass, and the eggs hatch into ‘tadpoles’ that wriggle about in the foam. However, if they do not eat anything but yolk, I would not call them tadpoles.
Overall, it is only really Arthroleptis that exemplifies the ‘salamander-equivalent’ among frogs in South Africa, in its clearest expression. But this is not trivial, because Arthroleptis can be the commonest vertebrate in its restricted habitats.
The equivalent of Arthroleptis in the Neotropical rainforests is the genus Eleutherodactylus (family Eleutherodactylidae). This contains 185 spp., all with direct development, and no aquatic stage.
In Tasmania, the ‘moss froglet’ (Myobatrachidae: Bryobatrachus) is the equivalent of South African Arthroleptella, and likewise develops out of the water. In the karri forest of southwestern Australia, the best example of Geocrinia (again, a member of the Myobatrachidae, which also contains Kyarranus, Myobatrachus and Arenophryne as examples of Australian frogs with direct development out of the water).
(writing in progress)
Comentários
Why are frogs would be more competitive in the salamander niche in southern locations? One clue is that the Northern Hemisphere has more extreme temperatures than the southern lands, which tend more to be tempered by proximity to the sea. The northernmost salamanders are remarkably adapted to cold, being able to locomote even on ice. Perhaps having a long tail somehow gives salamanders some degree of thermal intertia, not needed by like-size frogs which eat similar foods in adulthood and shelter in similar ways in the vegetation? The lunglessness of a major family of salamanders in North America, viz. Plethodontidae, may also partly be adaptive to cold, based on reducing internal surface areas?
Would be interesting to see if there is a relationship between direct developing frog species diversity and salamander diversity. It sounds like you are proposing that salamanders have been excluded and outcompeted by direct developing frogs? Might also be interesting to compare the age of direct developing frog clades between regions of the world with and without salamanders (if direct developing frogs have older clades than areas with salamanders).
Adicionar um Comentário